Some vehicle engine systems utilize both direct in-cylinder fuel injection and port fuel injection. The fuel delivery system may include multiple fuel pumps for providing fuel pressure to the fuel injectors. As one example, a fuel delivery system may include a lower pressure fuel pump (or lift pump) and a higher pressure fuel pump arranged between the fuel tank and fuel injectors. The high pressure fuel pump may be coupled to the direct injection system, upstream of a fuel rail to raise a pressure of the fuel delivered to the engine cylinders through the direct injectors. However, when the high pressure fuel pump is turned off, such as when no direct injection of fuel is requested, pump durability may be affected, as the pump may be mechanically driven by the engine crank or camshaft. Specifically, the lubrication and cooling of the pump may be reduced while the high pressure pump is not operated, thereby leading to pump degradation.
In one approach to reduce high pressure pump degradation, shown by Basmaji et al. in US 2012/0167859, the low pressure fuel pump and higher pressure fuel pump are operated depending upon engine conditions. For example, when direct injection is not needed and high pressure pump operation is not requested, the lower pressure pump is operated to maintain a fuel rail pressure in the fuel rail while supplying fuel to the engine through port injection. Operation of the higher pressure pump is then adjusted to maintain a high enough pump chamber pressure so that fuel is pushed through the piston-bore interface, thereby lubricating the pump. In this way, the approach of Basmaji provides zero flow lubrication of the pump. In addition to lubricating the higher pressure pump during zero flow conditions, the pump NVH characteristics are improved.
However the inventors herein have identified potential issues with the approach of US 2012/0167859. Zero flow lubrication may be limited in a dead zone of the high pressure fuel pump, the dead zone being a region of pump operation where a substantial change in the duty cycle of the pump does not lead to a substantial corresponding change in fuel rail pressure. Graphically, this range appears as a horizontal, or effectively horizontal, line between fuel rail pressure and pump duty cycle. It is noted that pump duty cycle refers to controlling the closing of the pump spill valve. For example, if the spill valve closes coincident with the beginning of the engine compression stroke, the event is referred to as a 100% duty cycle. If the spill valve closes 95% into the compression stroke, the event is referred to as a 5% duty cycle. While commanding a 5% duty cycle, in effect 95% of the displaced volume is spilled and the remaining 5% is compressed during the stroke.
While operating a high pressure pump in closed loop control within the dead zone, large amplitude limit cycling may occur. As fuel rail pressure decreases, the pump duty cycle increases but it has no substantial effect until it climbs above a threshold value (e.g. the end of the dead zone). The limit cycling occurs as a result of the delay in fuel rail pressure change during closed loop rail pressure control. In one example, during positive flow operation the target fuel rail pressure may decrease abruptly, causing the high pressure pump pumping rate to also decrease while in closed loop control. The reduction in pumping rate may cause the pump to operate in the dead zone. Without prior calculation of the dead zone, the feedback fuel rail pressure controller causes the aforementioned limit cycling. Operating in the pump dead zone wastes pump energy and reduces pump volumetric efficiency.
Thus in one example, the above issues may be addressed by a method for an engine fuel system comprising: decreasing fuel rail pressure below a threshold; then, while not direct injecting fuel into an engine, learning a dead zone for a high pressure fuel pump based on a change in pump duty cycle relative to a resulting change in fuel rail pressure; and while direct injecting fuel into the engine, adjusting the pump duty cycle to stay above the learned dead zone. In this way, fuel pump lubrication can be improved, even when operating in the dead zone.
For example, in an engine system that is fueled via both port and direct injection, a high pressure pump may be used for increasing fuel pressure in a rail connected to the direct injectors. In the same system, a low pressure pump may be connected upstream of the high pressure pump and provides pressure to the port injectors on a different rail in addition to providing fuel to the high pressure pump inlet. First, the fuel rail pressure is decreased to a low value by ceasing to pump and continuing to direct inject. Then, while not direct injecting fuel into the engine, such as when only port injecting fuel to the engine, the duty cycle of the high pressure pump may be incrementally changed in small amounts (e.g. 1%, 2%, 3%) and a resulting fuel rail pressure may be recorded. Once the fuel rail pressure increases based on the increase in duty cycle, then operation outside the dead zone is reached and the relationship between duty cycle and rail pressure can be learned. As an upper limit, the duty cycles stops incrementing when the rail pressure reaches a threshold, such as the fuel rail pressure relief valve setting. Based on the change in fuel rail pressure, a dead zone of the pump may be identified and a duty cycle transfer function may be adaptively updated. The transfer function may then be applied when direct injecting fuel into the engine to provide a duty cycle that allows pump operation outside the dead zone. In one example, the controller integral term would be limited such that the commanded duty cycle would not be less than the zero flow lubrication duty cycle corresponding to a particular fuel rail pressure. In effect, this involves commanding a minimum duty cycle that is always above and outside the adaptively learned dead zone.
In this way, by learning the relationship between duty cycle and rail pressure for a high pressure fuel pump, a dead zone of the pump may be accurately quantified so that the pump command can be adjusted in the dead zone. For example, the pump may be commanded to not operate in the dead zone. Alternatively, the pump may be commanded to operate at a fixed (e.g., minimum) duty cycle in the dead zone. By reducing pump operation in the dead zone, the time for pump response to rail pressure changes is improved, reducing pump limit cycling, particularly when operating the pump with closed-loop control. By allowing for improved zero flow lubrication, pump operation may be optimized to reduce degradation and increase the longevity of the high pressure pump. Overall, high pressure pump operation is improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.